Add GrContext api to update compressed backend textures.

Bug: chromium:1099255
Change-Id: I0c3f25ddb037e47e3b910fa89c3d8b3aa27b3114
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/302265
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
4 files changed